Attenuation on a Long Coated Pipeline with One Rectifier
A 12 km coated steel pipeline, 300 mm diameter, runs through soil averaging 4,000 ohm-cm. Coating conductance is estimated at 2.5e-6 S/m2. One impressed current rectifier and a single anode bed are installed at the midpoint. At commissioning, the potential at the rectifier drain point reads -1.20 V (CSE), but a survey at 3 km from the drain point reads -0.80 V and at 5 km reads -0.72 V. The owner's criterion is -0.85 V CSE. The design assumed uniform protection to the ends.
1. Explain why the potentials fall off and whether this is a coating problem or an attenuation problem.
Read the worked answer
This is classic attenuation on a coated pipeline. The pipe is a low-resistance conductor; the coating is a high-resistance path to earth. Current leaves the pipe along its length, so the pipe-to-soil potential decays exponentially with distance from the drain point. The characteristic length depends on the pipe's longitudinal resistance and the coating conductance.
The numbers fit attenuation, not a coating defect: the drop is smooth and progressive (1.20 -> 0.80 -> 0.72 V) rather than a sharp local dip, which is what a coating holiday would produce. A single coating fault would show a localized low reading and a steep local gradient, not a monotonic decay over kilometers.
So the design assumption of uniform protection was wrong. The fix is not more current alone: raising the drain-point potential to push the ends to -0.85 V would overprotect the midpoint badly (likely above -1.5 V, risking coating disbondment). The fix is to redistribute current sources.
2. What design changes would you recommend, and how would you check them?
Read the worked answer
Options, in order of preference:
- Add a second rectifier/anode bed at one or both ends, so each section is shorter and the attenuation length covers less distance. This is the standard fix for a long pipeline with a single midpoint source.
- Improve coating conductance by rehabilitating the coating, which lengthens the attenuation distance and reduces current demand.
- Use a distributed anode system (e.g., close-interval or linear anode) along the under-protected stretches.
Check the design with an attenuation calculation: estimate the characteristic length from pipe resistance and coating conductance, then confirm that the potential at the far end of each section stays at or above -0.85 V while the drain point stays within a safe overprotection limit. Verify with a post-installation close-interval survey and 100 mV decay tests at the ends and midpoint.
3. The owner asks whether simply raising rectifier output would solve it. What is your answer?
Read the worked answer
No. Raising output raises the drain-point potential, but the decay curve is exponential: to lift the far end from -0.72 to -0.85 V you would push the midpoint far above -1.2 V, into overprotection. That risks coating disbondment, hydrogen evolution, and wasted current. The correct response is to change the current distribution (more sources or better coating), not to increase the total current from one point.